Use of digital detectors, rather than radiographic film, to acquire X-ray image data is of steadily increasing interest. However, the dynamic range of image data provided by a digital detector is typically much greater than the dynamic range of devices used to display the data, such as printers and CRT monitors. Accordingly, the dynamic range of the image data supplied by a digital detector must be mapped to the dynamic range of the monitor or other display device. This is achieved by transforming the image data by means of a look-up-table (LUT). The transformation controls the xe2x80x9cbrightnessxe2x80x9d and xe2x80x9ccontrastxe2x80x9d of the displayed digital image, where brightness is synonymous with xe2x80x9cwindow centerxe2x80x9d and contrast is synonymous with xe2x80x9cwindow widthxe2x80x9d. More specifically, a digital X-ray detector can be a 14-bit device, that is, each detector pixel can provide a count value, representing data, which can be any number in a range between 0 and 16,383. On the other hand, display devices of the above type are typically 8-bit devices, so that they can output only 256 different gray scale levels, from white to black. However, even if the digital detector and display had the same number of bits, it could be desirable to apply transformation to enhance visualization of the areas of interest
At present, image-based techniques are commonly used to set the brightness and contrast of a digital image. For example, analysis of the histogram of a detected digital image may be used to segment the image into various areas, such as object and background. With the object identified, the mean count value in that area can be used to set the window center for display. The difference between the minimum and maximum count values in the object of interest may be used to set the window width. As is well known by those of skill in the art, the image of an object can be presented in multiple ways, by applying different transformations or LUTs to the detected data. One transformation would be a simple linear curve. More typically, the transformation would be defined by an S-curve of some form, which is generally linear along its center portion, has an associated slope, and is flattened at its ends.
Image-based techniques are highly dependent on a priori knowledge of the anatomy being imaged and on the robustness of the segmentation scheme being used. For example, image data acquired from a particular subject""s lungs could be used to develop a transformation algorithm, in order to construct an LUT. The mean count value could be taken from image data at the center of the lungs, and maximum and minimum count values could represent only a region immediately surrounding the lungs. However, while the algorithm may be useful in transforming data representing an image of another subject""s lungs, it would probably not be usable in connection with image data of other organs or body parts, or which represented a different background. Moreover, segmentation schemes are usually affected by the variation of count values, as well as their distribution in the image caused by the presence or absence of raw radiation, by devices such as pacemakers and collimators, and by grids.
In the invention, it has been recognized that X-ray imaging practitioners have had extensive experience in using analog X-ray film. Accordingly, the invention bases dynamic range management on a technique or procedure for acquiring X-ray image data, rather than on a particular image or object of imaging. Such method is considered to be much more adaptable to different imaging applications or requirements, and may also present images for viewing in a form which appears very similar to an analog film image.
The invention may be recited as a method for establishing or setting the dynamic range of a digital display device in an X-ray imaging system which comprises an X-ray source, a digital X-ray detector and the display device. In the method, the detector is operated to provide a set of count values representing X-ray image data acquired by the detector from an object of imaging. A specified analytical equation is applied to the set of count values, which transforms the count values to a range of standardized values. In a preferred embodiment of the invention, the standardized range of values represent optical densities. The method further comprises the step of mapping the standardized values to the dynamic range of the display device. The specified analytical function is derived from a particular count value, which uniquely corresponds to an X-ray dose required to provide a specified optical density in displaying image data on the display device. The function is further derived from the H-D curve of a specified analog film which has an associated gamma coefficient. Preferably, the specified optical density is 1.2.